Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 PM EST Mon Nov 26 2018

Valid 12Z Fri Nov 30 2018 - 12Z Tue Dec 04 2018

Models today have shown a moderate degree of improvement in
consensus throughout the extended period, particularly at larger
scales. Some significant points of contention remain when it comes
to the details, however. A strengthening Rex block across central
Asia during the extended period should promote increasingly
amplified flow across the North Pacific/Alaska. Teleconnections
associated with this feature support a building upper-level ridge
from the Gulf of Alaska north across the state, and this is indeed
what we see unfolding as we progress through the next week. The
result will be a westward shift in the dominant storm track from
what we have seen recently, toward the Aleutians and Bering Sea.
The potential continues to exist for phasing between multiple
streams and rapid deepening of cyclones, but the details of such
an interaction are often extremely difficult to forecast more than
a couple days out.

At the start of the extended period (day 4/Fri), the ECMWF and GFS
showed broad agreement on a low in the western Bering Sea and a
surface front crossing the Aleutians. The remnants of Tropical
Storm Man-yi will likely be absorbed into this frontal system,
with some potential phasing occurring Fri-Sat. Models have handled
this poorly, and the 12Z GFS/ECMWF both show a rapidly deepening
warm seclusion cyclone in the Bering Sea late Fri, a feature which
was not shown to near the same extent by previous runs of the same
models. Solutions have waffled back and forth on this over recent
days, it is likely prudent to hold off on adopting a solution with
such a deep cyclone until models show some consistency.
Regardless, the surface front should continue to sweep east toward
the Alaska Peninsula through Sat before weakening after moving
into southwest mainland Alaska Sat night. Additionally, models
have struggled with the amplitude of the intensifying upper ridge
Sat-Sun, and whether shortwave energy/westerly flow will be able
to make significant headway into south central Alaska. Models have
waffled on this as well, but the latest ECMWF reinforces the idea
(supported by the hemispheric pattern) that the ridge should
strengthen fairly quickly, not allowing significant shortwave
energy to move too far eastward. The GFS has trended a bit toward
the ECMWF in this regard over the past 24 hours, but still remains
more progressive, so some degree of compromise is likely warranted
here. Given these considerations, a blend of the 00Z ECMWF/06Z GFS
along with the ECENS/NAEFS means was used a starting point for the
WPC forecast during days 4-6.

From Sun onward, attention shifts west to the next energetic
shortwave crossing the North Pacific. Models/ensemble members
continue to demonstrate an unusually high degree of consensus
around this system, especially when the time frame (days 6-8) is
considered. ECMWF/GFS/CMC solutions all show a quickly amplifying
upper-level low by Sun night-Mon, with a surface low deepening
into the 950s (hPa). Ensemble members are also clustered fairly
well around these deterministic solutions, at least through day 7
(Mon) as the system crosses the western Aleutians and moves into
the Bering Sea. Confidence drops a bit by day 8 (Tue) as to
whether the low will move northeastward toward mainland Alaska, or
perhaps a new triple point low develops that will move east into
the Gulf. The large scale pattern would certainty seem to favor a
deeper/more consolidated system in the Bering Sea but will have to
wait on future model runs for more clarification on this.
Weighting of ensemble means was boosted in the forecast for days
6-8, but a minority (30%) of deterministic solutions was continued
through the end of the period given the moderate level of
consensus noted.


...Sensible Weather Highlights/Hazards...

Precipitation will affect the Aleutians and Alaska Peninsula Fri
as the front passes those areas, and precip will increase across
southern mainland Alaska by Sat-Sun as the front/shortwave reach
that area. The setup does not look overly favorable for widespread
heavy precip but locally heavy precip will be possible. the
Aleutians become active once again from Sun night onward as the
next low pressure system approaches. Given the expected intensity,
this system may also produce potentially high winds for portions
of the Aleutians and perhaps the Alaska Peninsula as well.
Gradually warming temperatures are possible across mainland Alaska
by late week as somewhat warmer Pacific air advects into the
region and the upper ridge begins to build in.


Ryan


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html